Mass flow meters for flow media that work on the Coriolis Principle are well known in various embodiments (see, for example, German Disclosure Documents 26 29 833, 28 22 087, 28 33 0379 29 38 498, 30 07 361, 33 29 544, 34 43 234, 35 03 841, 35 05 166, 35 26 2977 37 07 777, 39 16 285, 39 28 839, 40 16 907, 41 24 295 and 41 24 296, European Patent Disclosure Documents 0 083 144, 0 109 218, 0 119 638, 0 196 150, 0 210 308, 2 212 782, 0 235 274, 0 239 679, 0 243 468, 0 244 692, 0 271 605, 0 275 367, and 0 282 552, as well as U.S. Pat. Nos. 4,491,009, 4,628,744 and 4,666,421) and are increasingly being used in practice.
Mass flow meter for flow media that work on the Coriolis Principle are basically divided into those whose pipes are designed to be straight, and those whose pipes are designed to be curved--with single or multiple pipes--and as pipe loops. The mass flow meters in question are also divided into those with only one Coriolis pipe and those with two; in designs with two, they may be in series or in parallel fluidically.
Embodiments of mass flow meters in which the Coriolis pipe or pipes are designed to be straight are simple in mechanical design and consequently can be produced at relatively low cost. Also, the inner surfaces of the pipes are easy to work on , for example, to polish. They also have low pressure losses. The disadvantage is that at a certain construction or layout length, their natural frequency is relatively high. Embodiments of mass flow meters whose pipe or pipes are designed to be curved have disadvantages whereas those with a straight pipe or pipes have advantages; but their advantage is that at a certain construction length, their natural frequency is relatively low.
In a mass flow meter that works on the Coriolis Principle and has at least one basically straight Coriolis pipe, at a certain construction length, a relatively low natural frequency can be created, and at a certain natural Frequency, a relatively short construction length can be created, by having an oscillator that acts on the Coriolis pipe via a pendulum arm (see pending U.S. patent application Ser. No. 07/736,400, filed Jul. 26, 1991, claiming priority from German application P40 23 989.6 of Jul. 28, 1990). While the oscillator acts directly on the Coriolis pipe and thus excites the Coriolis pipe, at least almost exclusively, to bending oscillations in the commonly known mass flow meters with at least one basically straight Coriolis pipe, in the mass flow meter just described, where the oscillator acts on the Coriolis pipe via a pendulum arm, the Coriolis pipe is excited to torsion and bending oscillations. The point is that the natural frequency relevant for bending oscillations can be influenced, without influencing the length, the mass and/or the stiffness of the Coriolis pipe, namely by the pendulum arm, i.e., by the mass of the pendulum arm and by the distance between the longitudinal axis of the Coriolis pipe and the point where the oscillator acts on the pendulum arm. The aforesaid pending application discloses designs and advancements of the mass flow meter just described. To prevent repetition, therefore, reference will be made expressly to the content of U.S. patent application Ser. No. 07/736,400, which is hereby incorporated herein by reference.
Incidentally, in the mass flow meters that work on the Coriolis Principle that are known from the previous publications and have at least one basically straight Coriolis pipe, it is true that the Coriolis oscillations have a relatively low amplitude, so that only a very low measured value can be obtained. This is because of the relatively high stiffness of the straight Coriolis pipe, both in the excited mode and in the Coriolis mode.
In all the mass flow meters in question that work on the Coriolis Principle and have at least one basically straight Coriolis pipe, problems can also result from the fact that undesirable, i.e., interfering, oscillations can occur at frequencies that are relatively close to the frequencies of the desired oscillations, i.e., oscillations in the excitation mode and in the Coriolis mode.